1. Field of the invention
The present invention relates to a variable gain amplifying circuit for use with an amplification for amplifying a signal amplitude level.
2. Description of the Related Art
As a conventional variable gain amplifying circuit, a so-called Gilbert gain cell circuit as shown in FIG. 8 is known. In FIG. 8, signals are input to bases of a first transistor pair 1. Emitters of the first transistor pair 1 are connected to respective current sources 20 and 21. The emitters of the first transistor pair i are connected through a resistor 30. Collector currents of the first transistor pair 1 are used as bias currents of a first diode pair 11. The anodes of the first diode pair 11 are connected to a bias voltage source. Cathode voltages of the first diode pair 11 are applied to bases of a second transistor pair 2. Emitters of the second transistor pair 2 are connected as a common emitter to a current source 22. Current outputs are obtained from collectors of the second transistor pair 2.
In such a construction, the transconductance gain G between the input and the output is given by the following equation. EQU G=I2/(Re/2+re) I1 (1)
where Re is the resistance of the resistor 30; and I1 and I2 are current values of the current sources 20 and 21, respectively.
In addition, the following equation is given. EQU re=q.times.I1/kT (2)
where re is the emitter differential resistance of the first transistor pair 1; k is the Boltzmann constant; q is the charge amount of electrons; and T is the absolute temperature.
Since either the current value I1 or the resistance value Re increases, re can be ignored. Thus, the equation (1) can be represented by the following equation (3). EQU G=(2/Re) I2/I1 (3)
Consequently, when the ratio between the current values I1 and I2 is varied corresponding to the equation (3), the gain G can be freely changed. In addition, the emitter differential resistance re, which is a non-linear element, is omitted, it is clear that the linear characteristics of the input and output are good.
However, to remove the influence of re, it is necessary to increase the current value I1. This current increase prevents the circuit from being fabricated with an IC. If the resistance value Re is increased, the gain G decreases. To prevent the gain G from decreasing, the current value I2 should be increased, thereby preventing the circuit from being fabricated with an IC. Thus, the influence of re cannot be ignored. In other words, the linear characteristics of the input and output cannot be improved.
To solve such a problem, a circuit shown in FIG. 9 is known. In FIG. 9, the bases of the transistors of the first transistor pair 1 shown in FIG. 8 are connected to output terminals of operational amplifiers 40 and 41, respectively. Inverted input terminals of the operational amplifiers 40 and 41 are connected to the emitters of the transistors of the first transistor pair 1. Input signals are supplied to noninverted input terminals of the operational amplifiers 40 and 41.
Thus, a feed-back loop is constructed. An emitter differential resistance re of each transistor of the first transistor pair 1 is a inverce of the gain of each of the operational amplifiers 40 and 41.
Thus, when the gains of the operational amplifiers 40 and 41 are remarkably high, the influence of re can be ignored. The current value I1 and the resistance value Re should have an input dynamic range corresponding to the amplitude of the input signal. Consequently, since the current value I1 and the resistance value Re are decreased, the current consumption can be suppressed. As a result, the problem for fabricating the circuit with an IC can be solved.
However, since the frequency characteristics of the operational amplifiers are bad, the circuit shown in FIG. 9 cannot handle signals having relatively high frequencies. Thus, the applications of the circuit shown in FIG. 9 are limited.